Nowadays severe demands are made on fluid-actuated control systems, also referred to hereinafter as fluid medium systems, and especially on control systems of such type for automatic or automated transmissions in motor vehicles. Besides special requirements in relation to low weight at the same time as high reliability, long life and low maintenance, particularly in the case of passenger cars, vibrations and noise caused by the device constituents of the fluid medium system must be avoided or at least damped as much as possible. In addition, there are special requirements with regard to the actuation speed of control elements to be actuated since a traction force interruption caused by a gear change should last for as short a time as possible. At the same time, the fluid medium system should be produced and assembled as inexpensively as possible.
Thus until now, it has been usual in fluid-actuated transmissions, following a control action, i.e., a change in the position of at least one fluid-actuated control element, to switch off the pressure on the pressurized control element(s) or at least reduce it to a lower pressure sufficient for maintaining the desired position of the pressurized elements. This enables the load on the pressure medium lines and other pressurized parts of the pressure medium system and control elements to be relieved and, therefore, makes it possible to reduce the loads on the fluid medium system as a whole and correspondingly to design it weaker and lighter and/or increase its service life. In addition, if the system is only acted upon by high pressure while shift actions are actually taking place then, on account of the leak flows that always exist, this has advantages in relation to the energy required in order to produce the pressure and in relation to fluid loss if there are leaks in the system components concerned.
In order to prevent or at least reduce the rebound of a pressurized movable component of a control element, for example off a fixed stop abutment, and bring the pressurized movable component reliably to rest in a desired nominal position, it is usual, when the component has reached the desired position, for example against a mechanical stop abutment, to continue still acting upon it with pressure for a short time; the so-termed post-shifting time.
In the case of a fluid medium system with a plurality of control elements, it was sometimes regarded as advantageous, when a shift process or some other desired movement of at least one pressure-medium-actuated control element had to be carried out, for example to use a main valve to impose upon the system as a whole a standardized pressure for a standardized time consisting of a movement time and a post-shifting time, whereby the control element(s) to be actuated or their moving parts are moved to the desired actuation position while the other control elements or their moving parts are maintained fixed in their position by the action of the pressure.
Often, however, the control elements are also connected, via control valves fitted near them, to a pressure medium line that is kept continuously under a standardized operating pressure during operation and to a pressure medium outflow line under a lower pressure or to an unpressurized line so that, for the actuation of a control element, for example by corresponding valve actuation, the control element is selectively pressurized with the operating pressure for a standardized time. In the case of pneumatic systems, the pressure medium outflow line can often be replaced by simple means for venting to the surroundings. In this case as well, it is usual and has until now often been regarded as advantageous to provide a predetermined post-switching time.
To fulfil its function, in both cases the post-switching time should be chosen such that it can perform its intended purpose of damping rebound off stop abutments and securing the desired position of the pressurized moving parts of the control elements, so far as possible for all the control elements in the system and under all operating conditions.
However, the control method with a fixed post-switching time described above suffers from the following disadvantages:
Since the post-switching times required for the optimum operation of or desired for different control elements and locations thereof are very different, compromises are often necessary so that the post-switching times for one or some control elements have to be shorter or longer than actually needed.
The result is either that system components for which a shorter post-switching time would suffice are pressurized for an unnecessarily long time or else that system components which need particularly long post-switching times, or for which these would be desirable, are pressurized too briefly or at any rate for times that are not optimum under all operating conditions or not completely sufficient.
A design of the pressure medium system to give optimum post-switching times as similar as possible would, in most cases, result in disproportionate expense and/or a neglect of more important design objectives, such as simple and assembly-friendly line positioning and the use of lines with cross-sections as uniform as possible.
However, even if such a design were to achieve at least approximately equal post-switching times without neglecting other design objectives, a uniform post-switching time could at best only be optimized for a standard operating situation or for an extreme operating situation. Here the problem arises that an extreme operating situation demands a considerably longer, or perhaps a considerably shorter post-switching time than a standard operating situation and, therefore, either the post-switching time is set unnecessarily long or undesirably short during most of the operating time, or in extreme operating conditions the post-switching time is too short or too long.
Furthermore, with a system of the type usual until now it is as difficult to take into account different optimum post-switching times for different actuation directions of a control element, as it is to allow for differences in the definition of an optimum post-switching time determined with reference to different parameters.
For example, a minimum post-switching time that is still sufficient for moving a control element accurately enough to a nominal position and securing it there may be optimum in the event of a pressure drop in the fluid medium system, or other difficulties related to the pressure medium supply, whereas in the case of comfort-optimized shifting behavior of the transmission controlled by the fluid medium system, as desired by the driver, relatively long post-switching times have to be set in order to damp possible continuing oscillation of the moving parts of a control element as effectively as possible and to minimize pressure waves in the system and vibration caused by them.
In contrast, if the driver wants the shifting behavior to be made as sporty as possible, it can also sometimes be advantageous to have a relatively short post-switching time, so as to be able to bring the system back as quickly as possible to a defined starting condition and enable a repeated gear change. The resulting, slightly louder shift noise or vibrations may even be regarded as desirable in some cases, since they serve as indications of uncompromising design for power or sporty behavior.
Against that background the purpose of the present invention is to propose a control method for a fluid medium system, for example for an automated or automatic motor vehicle transmission, which is exempt from the disadvantages outlined above or at least substantially reduces them. A further objective is to describe a control system for implementing such a method.